1. Field of the Invention
The present invention relates to an electric device, in particular for medium or high voltage electric substations, able to perform the functions of circuit-breaking, isolating and earthing. In other words, the invention relates to a device suitable for installation in cubicles of the protected type which form medium or high voltage electric switchboards. According to one embodiment, the device of the invention has a resilient member for transferring correct pressing loads to the circuit-breakers. Furthermore, in the device according to one embodiment of the invention, a circuit-breaker actuating shaft and a line isolator actuating shaft are coaxial.
2. Related Art and Other Considerations
As is known, an electric substation receives voltage from voltage busbars and conveys current via a line directed to the user. Electric devices, typically (line and earthing) circuit-breaker and isolators, are provided between the voltage busbars and the line directed to the user.
A circuit-breaker for electric substations is able to protect the line directed to the user. A line isolator, after the circuit-breaker has opened the line directed to the user from the voltage supply busbars, is able to disconnect the line physically. Finally, an earthing isolator, again after the line isolator has disconnected the line directed to the user from the voltage supply busbars, earths the line directed to the user in order to avoid the occurrence of discharges or induced currents. The assembly formed by all the abovementioned devices, as well as other devices not relevant for the purposes of the present invention, which are present in an electric energy distribution node, is generally referred to by the term “switchboard”. Each switchboard is composed of several cubicles in each of which the circuit-breaking, isolating and earthing functions for each line are contained.
At present, in most of the cubicles which are commercially available, the same functions are performed using different apparatus for isolation, earthing and circuit-breaking. In particular, the circuit-breaker is usually a separate component.
The known cubicles generally have fairly large dimensions. Typically they have a height of between about 170 and 250 cm, a depth of about 100 cm and a width of between about 50 and 100 cm.
EP 1,226,596 in the name of the same Applicant describes a three-pole apparatus for electric substations, having an extremely compact structure compared to the known apparatus. The device according to EP 1,226,596 comprises a series of isolating devices which comprise at least one line isolator, a circuit-breaker and an earthing isolator. The circuit-breaker is sealed inside a vessel in a vacuum and is positioned in series with a line/earth isolator device which is movable between a first position where it connects said circuit-breaker to a contact of the voltage busbars and a second position where it connects the circuit-breaker to an earthing contact. According to EP 1,226,596, the line/earth isolator and the circuit-breaker are all contained in a metal casing and the metal insulators are arranged at the input and output terminals of the device.
The device according to EP 1,226,596 represented a notable improvement compared to the known devices, in particular because of its extremely small dimensions.
In the device according to EP 1,226,596, a single shaft (denoted by the reference number 10) actuates the circuit-breakers by means of a lever connected thereto. Moreover, another shaft (indicated by the reference number 9) actuates the line isolators by means of another lever connected thereto. The line isolator shaft is situated at a distance from the circuit-breaker shaft and this complicates the arrangement of the various mechanisms inside the casing of the device and prevents optimum use of the available space, which is restricted by the small dimensions of the device.
As is known, the movable contacts in a vacuum circuit-breaker for use in medium or high voltage substations must be moved with a high degree of precision and in a substantially instantaneous manner, i.e. within the space of a few milliseconds.
Another problem associated with the operation of vacuum circuit-breakers consists in the extremely high pressing loads. Typically these loads are in the region of 1000–1500 N or more. As mentioned above, in the device according to the patent EP 1,226,596, these loads are transferred by means of the lever connected to the shaft 10. In particular, the lever connected to the shaft 10 actuates a round conductor rod with which it is pivotally hinged and a small cylinder which comes into contact with the movable circuit-breaker contacts. The Applicant has ascertained that such an essentially rigid configuration is unable to transfer (and maintain) in a sufficiently reliable and precise manner the abovementioned pressing loads onto the movable circuit-breaker contacts. Furthermore, the Applicant has faced the problem to optimize the space inside the device and to provide an optimized arrangement within the device.
U.S. Pat. No. 6,362,444 discloses a gas insulating switchgear which is not adapted for use in modular cubicles of medium voltage switchboards. The switchgear according to U.S. Pat. No. 6,362,444 can not be connected to voltage supply busbars.
U.S. Pat. No. 4,225,763 describes means for suppressing contact-separation at the end of a vacuum circuit breaker closing operation. The vacuum circuit breaker according to U.S. Pat. No. 4,225,763 does not perform any line isolator and earthing isolator function.
DE 12 44 913 describes a pure vacuum switch for high voltage substations which does not perform any line isolator and earthing isolator function.
EP 0,737,993 describes an hybrid break device for high tension which does not perform any line isolator and earthing isolator function.
FR 2,839,193 describes a hybrid switching mechanism with a dielectric filled gas envelope (12) and a vacuum switch envelope. The mechanism does not perform any line isolator and earthing isolator function.
U.S. Pat. No. 4,713,503 describes a three-phase vacuum switch operating mechanism with anti-bounce device for interrupter contacts.